1. Field of the Invention
The present invention relates generally to connection apparatus for use with seismic braces and, more specifically, to nonrigid connection apparatus for seismic braces. In particular, the present invention relates to connection apparatus which prevent seismic and gravitational loads on steel structural frames from being applied nonaxially to seismic braces. By way of example, the connection apparatus may allow the brace to pivot relative to a structural frame.
2. Background of Related Art
In many areas of the world, particularly seismically active areas, large buildings and other structures may be subjected to seismic loads. In order to prevent structures from being damaged by seismic loads, particularly the vibrations that follow the application of seismic loads to structures, or to at least reduce the amount of damage that seismic loading may cause to such structures, various shock-absorption devices have been developed.
One such shock absorption device, which is useful with steel structural frames, is commonly referred to as a “seismic brace.” As shown in FIG. 1, a pair of seismic braces 10 is often arranged within each “bay” 32 of a steel structural frame 30, each bay 32 typically being formed by an adjacent pair of substantially horizontally oriented steel beams 34 (e.g., beams 34u, 34L shown in FIG. 1) and an adjacent pair of substantially vertically oriented steel columns 36. Bottom corners 38 of each bay 32 are formed at junctions between a lower substantially horizontally oriented steel beam 34L and the substantially vertically oriented steel columns 36 at each side of bay 32. Lower ends 12L of seismic braces 10 are typically secured at opposite bottom corners 38 of bay 32. Upper ends 12u of seismic braces 10 are typically secured to an upper substantially horizontally oriented steel beam 34u at adjacent, substantially central locations thereof. As such, the two seismic braces 10 within a bay 32 of steel structural frame 30 are arranged in an inverted “V” configuration. Other, similar arrangements of seismic braces are also known, including “V” configurations, alternative “V” and inverted “V” configurations, a single, diagonally oriented seismic brace 10 in each bay 32 and another, oppositely oriented seismic brace 10 in the next laterally adjacent bay 32 (i.e., such that seismic braces 10 in two adjacent bays 32 form a “V” or inverted “V”), and the like. By arranging seismic braces 10 in this manner, when a seismic, or earthquake, load is applied to the structure of which steel structural frame 30 is a part, typically by shearing bay 32 in the directions of arrows 40 and 42, one seismic brace 10a of a pair will be subjected to a compressive load, depicted by arrows 44, while a tensile load, illustrated by arrows 46, will be applied to the other seismic brace 10b. 
Conventionally, seismic braces have been rigidly secured to the beams 34 and/or columns 36 of steel structural frames 30. FIGS. 2 through 2B illustrate an exemplary conventional connection, which includes the use of planar gusset plates 15 that are welded into place relative to a beam 34 and/or a column 36 and which have perpendicular extensions 16 welded to each side thereof. As shown in FIG. 2A, a cross-section taken perpendicular to the planes of both gusset plate 15 and extensions 16 thereof has a generally cruciform shape and, thus, four interior corners 17. Thus, each gusset plate 15 is configured complementarily to the exposed end 12 of a yielding core 11 (FIG. 1) of a seismic brace 10 (FIG. 1), which also typically has a cross-section, taken transverse to the length thereof, that is generally cruciform in shape and, thus, includes four interior corners 13 that extend along the length thereof, as shown in FIG. 2B. The cross-section of an exposed end 12 of a yielding core 11 of a seismic brace 10 and the corresponding features of the cross-section taken through gusset plate 15 and extensions 16 thereof may have substantially the same dimensions. A rigid connection between these two elements is typically effected by way of intermediate securing elements 19, which are typically referred to as “splice plates,” positionable across portions of both an exposed end 12 and a gusset plate 15/extension 16, within corresponding interior corners 13 and 17. Each intermediate securing element 19 includes apertures 20, 21 formed therethrough, which respectively align with corresponding apertures 14 formed through exposed ends 12 of yielding core 11 and apertures 18 formed through gusset plate 15 and extensions 16 therefrom. Apertures 14, 18, 20, and 21 are typically configured to receive bolts 22, which, along with complementarily threaded nuts 23, secure intermediate securing elements 19 in place with respect to both gusset plates 15 and exposed ends 12 of yielding core 11, thereby securing seismic braces 10 into place relative to steel structural frame 30.
A seismic brace 10 (FIG. 1) is secured to a steel structural frame 30 by aligning exposed ends 12 of a yielding core 11 (FIG. 1) of each seismic brace 10 with a corresponding gusset plate 15 that has already been secured to one or more of a beam 34 and/or a column 36 of steel structural frame 30, as well as with extensions 16 that have been secured to that gusset plate 15. Intermediate securing elements 19 are then positioned within interior corners 13 and 17, then bolted (e.g., with bolts 22 and complementarily threaded nuts 23) to gusset plate 15, extensions 16 therefrom, and exposed end 12. As shown, the connection of exposed ends 12 to gusset plate 15 is typically established by way of four intermediate securing elements 19 which have L-shaped cross-sections, taken transverse to the lengths thereof.
Referring again to FIG. 1, in addition to applying loads axially to seismic braces as a result of the shear generated by seismic and gravitational loads, rigid connections of this type typically transfer additional shears and moments, which are generated as a seismic brace 10 drifts laterally. Application of shear and moment to a yielding core 11 of a seismic brace 10 along vectors which are not located in a plane of bay 32 undesirably causes a bending moment and shear stress to be applied to yielding core 11, which, along with compressive loads applied thereto, results in a so-called “combined stress” that is greater on one side of yielding core 11 than on the other and that may cause seismic brace 10 to buckle in an unintended direction. When such buckling occurs, seismic brace 10 is no longer useful for either shock absorption or structural support.
Thus, a connection apparatus which substantially isolates a seismic brace from nonaxially oriented loads, as well as that reinforces or isolates the seismic brace from shears and moments that occur as a seismic brace drifts from a plane of a bay of a steel structural frame in which the seismic brace is located, would be an improvement over the existing art of which the inventors are aware.